Aromatic polyester polyols are commonly used intermediates for the manufacture of polyurethane products, including flexible and rigid foams, polyisocyanurate foams, coatings, sealants, adhesives, and elastomers. The aromatic content of these polyols contributes to strength, stiffness, and thermal stability of the urethane product.
Commonly, the aromatic polyester polyol is made by condensing aromatic diacid, diesters, or anhydrides (e.g., terephthalic acid, dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. These starting materials usually derive exclusively from petrochemical sources.
As companies increasingly seek to offer products with improved sustainability, the availability of intermediates produced from bio-renewable and/or recycled materials becomes more leveraging. However, there remains a need for these products to deliver equal or better performance than their traditional petroleum-based alternatives at a comparable price point.
Bio-renewable content alone can be misleading as an indicator of “green” chemistry. For example, when a food source such as corn is needed to provide the bio-renewable content, there are clear trade-offs between feeding people and providing them with performance-based chemical products. Additionally, the chemical or biochemical transformations needed to convert sugars or other bio-friendly feeds to useful chemical intermediates such as polyols can consume more natural resources and energy and can release more greenhouse gases and pollutants into the environment than their petro-based alternatives in the effort to achieve “green” status.
Waste thermoplastic polyesters, including waste polyethylene terephthalate (PET) streams (e.g., from plastic beverage containers), provide an abundant source of raw material for making new polymers. Usually, when PET is recycled, it is used to make new PET beverage bottles, PET fiber, or it is chemically transformed to produce polybutylene terephthalate (PBT). Other recycled raw materials are also available. For example, recycled propylene glycol is available from aircraft or RV deicing and other operations, and recycled ethylene glycol is available from spent vehicle coolants.
Urethane formulators demand polyols that meet required specifications for color, clarity, hydroxyl number, functionality, acid number, viscosity, and other properties. These specifications will vary and depend on the type of urethane application. For instance, rigid foams generally require polyols with higher hydroxyl number than the polyols used to make flexible foams.
Polyols suitable for use in making high-quality polyurethanes have proven difficult to manufacture from recycled materials, including recycled polyethylene terephthalate (rPET). Many references describe digestion of rPET with glycols (also called “glycolysis”), usually in the presence of a catalyst such as zinc or titanium. Digestion converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers. Although such mixtures have desirably low viscosities, they often have high hydroxyl numbers or high levels of free glycols. Frequently, the target product is a purified bis(hydroxyalkyl) terephthalate (see, e.g., U.S. Pat. Nos. 6,630,601, 6,642,350, and 7,192,988) or terephthalic acid (see, e.g., U.S. Pat, No. 5,502,247). Some of the efforts to use glycolysis product mixtures for urethane manufacture are described in a review article by D. Paszun and T. Spychaj (Ind. Eng. Chem. Res. (1997) 1373.
Most frequently, ethylene glycol is used as the glycol reactant for glycolysis. This is sensible because it minimizes the possible reaction products. Usually, the glycolysis is performed under conditions effective to generate bis(hydroxyethyl) terephthalate (“BHET”), although sometimes the goal is to recover pure terephthalic acid. When ethylene glycol is used as a reactant, the glycolysis product is typically a crystalline or waxy solid at room temperature. Such materials are less than ideal for use as polyol intermediates because they must be processed at elevated temperatures. Polyols are desirably free-flowing liquids at or close to room temperature.
Dimer fatty acids (also called “dimerized fatty acids” or “dimer acids”) are chemical intermediates made by dimerizing unsaturated fatty acids (e.g., oleic acid, linoleic acid, ricinoleic acid) in the presence of a catalyst, such as a bentonite or montmorillonite clay. Commercially available dimer fatty acids are usually mixtures of products in which the dimer acid predominates. Some commercial dimer acids are made by dimerizing tall oil fatty acids. Dimer fatty acids are commonly used to synthesize polyimide resins used in inks and hot-melt adhesives (see, e.g., U.S. Pat. No. 5,138,027). They are also components of alkyd resins, adhesives, surfactants, and other products.
Less commonly, dimer fatty acids are used as urethane components, particularly when the urethane includes a recycled PET-based polyol. One exception is JP 2004-307583, which describes a method for producing a polyester polyol and cured polyurethane. The '583 publication describes a two-step method in which recycled PET is digested with a glycol in the presence of a transesterification catalyst. The resulting product is then reacted with a polybasic acid having 20 or more carbons and no polymerizable double bond. Dimer acids are taught as suitable polybasic acids for the second step. The reaction product is subsequently reacted with MDI to make a simple urethane coating. In the working examples, a relatively large proportion of dimer fatty acid is used (one or more equivalents of dimer acid per equivalent of recycled PET), and it is unclear whether satisfactory results could be obtained with less dimer fatty acid. The large proportion of dimer fatty acid also severely limits the amount of recycle content (rPET plus any recycled glycol) in the polyol.
Improved polyols are needed. In particular, the urethane industry needs sustainable polyols based in substantial part on recycled polymers such as the practically unlimited supply of recycled polyethylene terephthalate. Polyols with high recycle content that satisfy the demanding color, clarity, viscosity, functionality, and hydroxyl content requirements of polyurethane formulators would be valuable.